In typical heat exchangers, water condenses out of the surrounding air onto the “cold” coils or surfaces of the heat exchanger. The condensed water has a tendency to “bead up” on the exchanger heat transfer surfaces as a result of the generally prevailing hydrophobic properties of the heat exchanger material, typically aluminum, and as a result considerable water is held in the exchanger, reducing heat transfer efficiency. Additionally, the trapped water eventually evaporates back into the surrounding air during normal cycling, raising the humidity of the surrounding air. It is even possible under mild temperature conditions or a low refrigerant charge an air conditioning system, to actually raise the relative humidity of an enclosed structure in which the condensing unit is housed.
The present invention is directed to providing hydrophilic surfaces for heat exchanger coils, fins, and similar heat transfer surfaces. By making the heat exchanger surfaces hydrophilic, moisture condensed from the surrounding air wets the hydrophilic surface immediately and does not build up on the heat exchanger surfaces. Consequently, the condensed moisture rapidly drains from the heat transfer surfaces, improving heat transfer efficiency and reducing the amount of water available to re-evaporate into the surrounding air.
It has long been recognized that hydrophilic heat exchanger surfaces are desirable. For example, U.S. Pat. No. 4,664,182, issued May 12, 1987 to Miwa Kazuharu for HYDROPHILIC FINS FOR A HEAT EXCHANGER, describes a hydrophilic coating comprised of gelatin and a water soluble acrylic resin. However, such coatings are subject to deterioration when exposed over a period of time in a high-moisture environment. More recently, U.S. Pat. No. 5,514,478 was issued on May 7, 1996, to Sadashiv K. Nadkarni for NON-ABRASIVE, CORROSION-RESISTANT, HYDROPHILIC COATINGS FOR ALUMINUM SURFACES, METHODS OF APPLICATION, AND ARTICLES COATED THEREWITH. The Nadkarni patent describes a non-oxide, i.e., non-ceramic coating and therefore lacks the hardness, abrasion resistance, and wear resistance of a ceramic coating.
Also, in the field of nanotribology, it is known that the macroscopic laws governing friction are inapplicable to microscale patterned surfaces. More specifically, increased lubrication properties have been found with nanoscale structured surfaces. However, uniformly textured nanoscale surfaces have heretofore been difficult to form on a consistent and durable basis.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a metal oxide coating that has hydrophilic and oleophilic surface properties. It is also desirable to have such a coating that has a uniformly patterned nanotextured surface having increased wetting and lubrication properties. It is also desirable to have a method for forming such a coating that is carried out at low temperature, is economical, and environmentally benign. It is also desirable to have such a method for forming patterned hydrophilic and oleophilic coatings that is able to coat complex shapes, and porous as well as non-porous materials, uniformly. Further, it is desirable to have such a method that is compatible with a wide range of substrates, including metals and metal oxides, as well as plastics and other temperature-sensitive materials.